1. Introduction
Coal is a key energy source and a major fuel of global economic growth. Efficient coal production relies heavily on the implementation of effective transportation systems. Achieving the criteria of efficient and safe transportation is a significant challenge for traditional mining transportation vehicles [
1,
2]. The importance of developing intelligent transportation technology cannot be overstated when it comes to enhancing the efficiency of coal transportation. In June 2021, the National Energy Administration and the National Mine Safety Supervision Bureau collaborated to release the “Guidelines for the Intelligent Construction of Coal Mines.” These guidelines aim to facilitate the systematic and organized implementation of intelligent construction practices across the coal mining industry [
3]. The auxiliary transportation vehicles usually employed in underground coal mines are regarded as one of the most frequently utilized means of transportation [
4]. Electric motor-based transportation plays an essential role not only as an integral component of the auxiliary transportation system in coal mines but also as a significant facilitator in supporting the regular operation of mining equipment and enhancing overall production efficiency [
5,
6]. In this regard, it is necessary to develop new tools to investigate the transportation system in coal mines further.
The complex task of navigating and parking an automobile within limited places is a considerable obstacle, even for highly skilled individuals operating the vehicle. Although driverless cars have shown competence in traversing highways, the ability to park in tight areas continues to pose a significant challenge [
7,
8,
9]. The act of perpendicular parking requires the ego vehicle to reverse its direction of motion and perform precise maneuvers to align with the parking spot [
10,
11]. In the middle of the century, Ishikawa introduced the concept of an optimum control model to determine the speed profile. This method is applied to both urban rail transit and railway systems. In their work, the authors of Reference [
12] investigated balancing energy consumption and comfort while solving the mixed-integer linear programming problem using pseudo-spectral methods. Moreover, the study referenced as [
13] examined a train timetable problem to minimize both the overall journey time and energy consumption. In a study referenced in [
14], energy consumption was quantified using an integral representation. In the existing literature, mostly electric locomotives mainly rely on the driver’s driving experience to stop at the target position and minimize the cost energy, and when the driver operates improperly, the actual stopping position will have a large deviation from the predetermined position, which seriously affects the efficiency of automated transport in underground coal mines, and is not conducive to the realization of intelligent and unmanned auxiliary transport in coal mines. The control and dispatch of electric locomotives within a transportation system directly impact transport efficiency and costs [
15,
16,
17]. In addition, the working conditions in underground coal mines are complicated, and different speeds, slopes, loads, and distances will affect the actual braking force of the motor vehicle and the driver’s stopping operation, which increases the difficulty of stopping [
18]. Therefore, the study of a locomotive parking control method that can effectively and autonomously control the locomotive to accurately stop at the target position within the allowable range of error is of great significance to the realization of intelligent and unmanned auxiliary transportation systems in underground coal mines and is also the development trend of the future intelligent coal mines [
19]. Developing a locomotive parking control method is crucial in automating auxiliary transportation vehicles [
20]. With profound implications, this research aims to achieve precise parking for tracked vehicles. Firstly, precise parking is a prerequisite for intelligent and unmanned underground coal mine transportation, enhancing efficiency and enabling automated production. Secondly, it is vital for the safety of coal mine transportation systems. Locomotives must execute precise parking functions to prevent accidents and optimize safety. There is room for further investigation, which will addressed carefully in this research.
Conventional motor trucks rely on DC and three-phase asynchronous motors, which have drawbacks like electric sparks and low efficiency. PMSM (permanent magnet synchronous motor) electrical actuators are widely used in many applications owing to their exceptional performance characteristics. These include a higher steady-state torque than induction machines, a more straightforward controller design for the PM motor, a better power density, and improved efficiency resulting from decreased rotor losses. In contrast, PMSMs offer a compelling solution: smaller, more reliable, and efficient, with high power density and low noise. Their adoption promises increased safety and energy savings, representing the future of mining locomotives [
21,
22,
23,
24]. This research focuses on developing a parking control method for underground coal mine electric locomotives using PMSMs. The objective is to enable precise parking under diverse conditions, aligning with the direction of intelligent and unmanned auxiliary transportation systems in coal mines. By addressing the challenges and harnessing PMSMs’ advantages, this research contributes to safer and more efficient electric locomotive operations, crucial for the future of coal mining. This paper proposes a positive solution to address the challenges posed by excellent control performance with a wide speed control range based on the inductance of PMSMs at low speeds.
Researchers have explored various advanced control algorithms to address the complexities of autonomous parking. Backstepping (BSC), Sliding Mode (SMC), Feedback Linearization (FBLC), Passivity-Based (PBC), and H1 control, along with optimal control techniques like Model Predictive (MPC), LQR, LQI, and LQG, have been proposed to tackle the challenges of autonomous parking. Adaptive controllers, including direct or indirect MRAC, parameter variation control (PVC), Extended Method (EMC), observation and estimation approaches, and intelligent control techniques such as Fuzzy logic control (FLC), ANFIS, and other variants, have also been investigated. Feedback linearization (FBL) is a control technique that simplifies the design of controllers by transforming the nonlinear system dynamics into a linear system. This approach ensures global stability and facilitates the development of more effective control strategies. Vector-based control techniques based on feedback linearization have emerged as innovative and promising solutions for complex control problems [
23,
25,
26,
27,
28]. Backstepping control offers an alternative design methodology for the feedback control of uncertain nonlinear systems. Backstepping control facilitates the design of effective controllers by identifying the state variables, inputs, and outputs of the nonlinear model. Additionally, it utilizes a Lyapunov candidate function (CLF) to analyze the system’s stabilization rigorously. A recent study has compared the robustness of H1 robust control and sliding mode control (SMC) for PMSM applications. Similarly, research has explored the intelligent control of PMSM using a fuzzy-based multi-variable optimization approach [
29,
30,
31,
32]. However, a comparative analysis of nonlinear vector-based controllers for PMSM remains lacking. This research focuses on the PMSM, recognizing it as the critical dynamic component within the actuation system that demands meticulous design consideration. To address this gap, we introduce a novel F-PID controller, a promising candidate for expanding the horizons of the power sector.
In this research, the authors focus on developing a parking control method for underground coal mine electric locomotives using PMSMs. Firstly, the main objective is to enable precise parking under diverse conditions, aligning with the direction of intelligent and unmanned auxiliary transportation systems in coal mines. Secondly, to tackle the challenges and issues, a new PMSM method has been introduced to get the optimized performance as far as this research contributes to safer and more efficient electric locomotive operations, which is necessary for the future of coal mining. Lastly, an experiment was presented to show the effectiveness of our proposed algorithm.
2. Mathematical Model
When the locomotive is stationary, it is essential to regulate the PMSM to provide the appropriate braking torque based on its present speed, distance, slope, and load. Hence, to enhance parking precision, this part examines the mathematical model of the PMSM, simplifies its control model, and derives the locomotive’s control strategy.
An inaccurate or slow response of the brake torque will harm the precision of parking the locomotive. Therefore, it is necessary to develop an appropriate control strategy for the PMSM to enhance the accuracy and stability of parking the locomotive. This section primarily presents the double closed-loop vector control method and F-PID control algorithm for the PMSM. It utilizes the double closed-loop vector control method based on F-PID to regulate the PMSM.
As shown in
Figure 1, the three-phase stationary coordinate system can be converted to two-phase stationary coordinate system by Clark transformation, and the two-phase stationary coordinate system can be converted to two-phase rotating coordinate system by Park transformation. When studying the PMSM, the first step is to establish a mathematical model under the three-phase stationary coordinate system of the motor and stator, and the three coordinate axes ABC take the direction of the magnetic field generated by the three-phase stator windings of the PMSM as the reference line.
In the three-phase stationary coordinate system, the stator voltage equation of the PMSM is:
Where: - current phase A, - current phase B, - current phase C, - winding resistance, - phase A stator magnetic chain, - phase B stator magnetic chain, - phase C stator magnetic chain.
The electromagnetic torque of the motor is:
Where: p-differential operator, - motor’s induced electromotive force on axes A, B and C, - three-phase stator self-inductance harmonic mean value, - angular velocity of the motor.
Figure (b) depicts the connection between the ABC coordinate system and the α-β coordinate system, with the Clark transformation facilitating the conversion from ABC to α-β coordinates. Utilizing the depicted relationship in Figure (b), the transformation formula between the two coordinate systems can be derived as follows:
The chain-voltage relationship of the PMSM in the α-β coordinate system can be expressed as:
Where: - phase α stator magnetic chain, - phase β stator magnetic chain.
Therefore, the torque of the PMSM in the α-β coordinate system is:
The process of transforming an α-β stationary coordinate system into a d-q rotating coordinate system, as shown in
Figure 1(b), is called the Park transformation.
According to the interrelationship between the two coordinate systems shown in
Figure 1(b), the following equation can be obtained:
Where current on d and q axis
In the
d-q coordinate system, the voltage of a
PMSM can be expressed as:
Where: - d-axis magnetic chain, - q-axis magnetic chain in d-q coordinate system.
Therefore, the electromagnetic torque of the
PMSM is:
2.1. SVPWM Technology
The core principle of Space Vector Pulse Width Modulation (SVPWM) involves managing the inverter’s output of three sinusoidal voltage signals by employing distinct combinations and sequences of activation and deactivation. These actions directly influence the stator windings of the PMSM. The resultant vector voltage, denoted as , undergoes rotation along a predetermined path in accordance with the three-phase voltage signals. This rotational control effectively governs the PMSM in the locomotive, ensuring precise control over the motor to produce the desired torque output.
Figure 2.
Linear combination of voltage space vector figure.
Figure 2.
Linear combination of voltage space vector figure.
In
Figure 1, the synthesis of voltage vectors is depicted. Focusing on sector 1, the resultant output voltage (
) is composed by combining non-zero vector voltages
(1 1 0) and
(1 0 1) with the zero vector
(1 0 0) from two neighboring regions, accounting for their respective durations of influence.
Where: - modulation time, - action time of , - action time of , - action time of , - output voltage, - DC bus side voltage, - RMS value of phase voltage, - the angle between the principal vector and the synthesized vector.
2.2. Vector Control of PMSM
The control strategy of
PMSM is investigated as it is controlled to generate braking torque and control the locomotive parking by vector control method during locomotive parking.
Where: - torque of motor, - ratio of locomotive transmission, - main reduction gear ratio of locomotive, - mechanical efficiency, - air resistance coefficient, A - front projection area, - traveling speed. G - gravity force on the locomotive (including self-weight and load), α - inclination angle, f - rolling resistance coefficient of the track, r - radius of the locomotive wheels, δ - rotating mass conversion factor of the locomotive, m - mass of the locomotive x - distance from target, v - different speeds.
q-axis current of the motor:
As can be seen from Eq. 10, the braking torque output from the PMSM can be controlled by controlling the q-axis current of the motor, which, combined with the parking control method, realizes the parking of the locomotive under different initial speeds, distances, slopes, and loads in the underground of the coal mine.
Figure 3.
Schematic diagram of double closed-loop vector control of PMSM.
Figure 3.
Schematic diagram of double closed-loop vector control of PMSM.
2.3. F-PID Vector Control of PMSM
The PID control methodology finds application in the locomotive parking process, showcasing its widespread utility in the realm of motor control. When overseeing the operation of the PMSM for locomotive propulsion, the initial step involves establishing the target torque value () for the PMSM within the PID control system. Subsequently, the actual torque () output of the motor is periodically measured and compared against the designated torque value (). The resulting deviation between the set and actual torque values is then input into the PID controller, which has been fine-tuned with proportional, integral, and derivative coefficients. This input enables the PID controller to generate the necessary output, thereby completing the control of the entire system.
The transfer function of the PID is:
Where: , , are proportional, integral and differential time constants, respectively.
The differential regulator can be likened to incorporating a damper into the system, effectively mitigating overshooting and enhancing system stability. A substantial differential time constant contributes to slowing down the system’s response. Hence, it becomes imperative to judiciously determine the values of the three parameters , , and in accordance with specific situations.
In the control system, fuzzy rules are primarily based on the deviation (e) between the given torque () and the actual output torque () of the permanent magnet synchronous motor. For significant errors (large e), increasing speeds up system convergence, while reducing Kd mitigates overshooting. Moderate errors (medium e) call for an intermediate value of to prevent substantial overshooting during system stops. Small errors prompt a focus on control system stability, suggesting a smaller value. Fuzzy control defines subsets for torque deviation (e) and deviation rate (ec) as {NB, NM, NS, ZO, PS, PM, PB} corresponding to {negatively large, negatively medium, negatively small, zero, positively small, positively medium, and positively large}. Post-processing maps these subsets to adjust the PID’s three-time constants within the range [-6, 6].
Table 1.
Fuzzy Rules.
Table 1.
Fuzzy Rules.
ec |
e |
NB |
NM |
NS |
ZO |
PS |
PM |
PB |
NB |
PB |
PB |
PM |
PM |
PS |
PS |
ZO |
NM |
PB |
PB |
PM |
PM |
PS |
ZO |
ZO |
NS |
PM |
PM |
PM |
PS |
ZO |
NS |
NM |
ZO |
PM |
PS |
PS |
ZO |
NS |
NM |
NM |
PS |
PS |
PS |
ZO |
NS |
NS |
NM |
NM |
PM |
ZO |
ZO |
NS |
NM |
NM |
NM |
NB |
PB |
ZO |
NS |
NS |
NM |
NM |
NB |
NB |
Table 2.
Fuzzy rules.
Table 2.
Fuzzy rules.
ec |
e |
NB |
NM |
NS |
ZO |
PS |
PM |
PB |
NB |
NB |
NM |
NM |
NS |
ZO |
ZO |
ZO |
NM |
NB |
NM |
NM |
NS |
ZO |
ZO |
ZO |
NS |
NM |
NM |
NS |
ZO |
ZO |
PS |
PS |
ZO |
NM |
NS |
NS |
ZO |
PS |
PS |
PS |
PS |
NM |
NS |
ZO |
PS |
PS |
PM |
PM |
PM |
NS |
ZO |
ZO |
NS |
PS |
PM |
PB |
PB |
ZO |
ZO |
PS |
PS |
PS |
PB |
PB |
Table 3.
Fuzzy rules.
Table 3.
Fuzzy rules.
ec |
e |
NB |
NM |
NS |
ZO |
PS |
PM |
PB |
NB |
PS |
PS |
ZO |
ZO |
ZO |
PM |
PB |
NM |
NS |
NM |
NM |
NS |
PM |
PM |
PM |
NS |
NB |
NM |
NM |
NS |
PM |
PS |
PM |
ZO |
NM |
NM |
NM |
NS |
PS |
PS |
PM |
PS |
NB |
NB |
NS |
NS |
PS |
PS |
PM |
PM |
NM |
NB |
NS |
NS |
ZO |
PM |
PB |
PB |
PS |
ZO |
ZO |
ZO |
ZO |
PB |
PB |
Finally, three surface models with time constants varying with e and ec are obtained, as shown in
Figure 4.
2.4. F-PID Vector Controller in Parking System
This section employs
F-PID double closed-loop vector control to govern the
PMSM. The system dynamically adjusts PID parameters based on current working conditions, showcasing excellent adaptability. Utilizing the fuzzy control system, the
PMSM swiftly and accurately produces braking torque under varied working conditions, enhancing locomotive stopping precision.
Figure 5 (a) illustrates the established
F-PID control system in this study.
In electric locomotive deceleration, a double closed-loop vector control system manages the torque and current rings to enhance control precision and minimize stopping errors in the PMSM. Introducing a F-PID controller to the outer torque ring facilitates fuzzy control of the torque. The parking process involves the underground coal mine locomotive parking system calculating the target torque () based on factors like speed, distance, gradient, and load. After comparing the designated torque () with the actual torque (), the torque deviation (e) and deviation rate (ec) are input into the F-PID controller. This controller processes the q-axis current () to simultaneously regulate the d-axis current () and the motor’s d-axis deviation rate (ec). The transformed d-axis and q-axis currents, driven by the PWM signal from SVPWM, achieve precise parking control.
In the parking process, as illustrated in Figure (b), the locomotive’s current speed, distance, slope, and load are analyzed using the parking control method. This analysis determines the appropriate braking torque, which is then supplied to the motor’s F-PID double-closed-loop vector control system. This control system effectively manages the PMSM to generate the designated braking torque, ensuring precise parking of the locomotive.